Clean up checked map array warnings and tests

Summary: This diff does the following:

-- Clean up warning messages to remove "converting to array" when it doesn't
-- Merge kExists* and kNvGet* warning message and change wording
-- Add take_by_ref.php test for msarray to match miarray

Reviewed By: @paroski

Differential Revision: D1488770
Remove dead code: defClsHelper

Summary: I looked quite a ways back and didn't find the last use being
removed; meh.

Reviewed By: @mxw

Differential Revision: D1488744